What do you expect how siglent react if you must send your in such manner modified scope for repair under warranty ?
I won´t stop the party and chances are high that siglent won´t reject the warranty in such cases, but it´s not that easy to expect it´s OK if you do it so without paying for it. 
I'd expect them to repair/replace if the problem had nothing to do with the keys.
But I'd also expect that if they refused I'd be left in a situation where I could raise a stink, but anything too far beyond that would likely be more than $400 worth of trouble.
That's the same situation I'd be in if I sent in a non-hacked scope for warranty work and they refused to honor the warranty. I don't know if Siglent would do such a thing (I'm guessing not, then again what do I know?), but in general the practice of refusing to repair/replace for whatever reason or making the warranty process costly or difficult enough for it to be not worth the trouble is not uncommon. Maybe you happened to catch an unhelpful customer service rep? Maybe you called during the wrong phase of the moon? At some point in anyone's life you'll be treated unfairly and fighting it will be more trouble than it's worth.
On the other hand, I'll even go so far as to admit that there's the possibility that I'm simply wrong (a very slight possibility

)
Each person has to gauge their own risk tolerance for such things.
But I'll wrap this up with a statement that I'm sure that the "warranty void if removed" sticker is a bogus scare tactic (at least in the US).